This application is based upon, claims the benefit of priority of, and incorporates by reference the contents of prior Japanese Patent Application No. 2001-315828 filed Oct. 12, 2001.
1. Field of the Invention
The present invention relates to a valve opening and valve closing timing control device for use in internal combustion engines.
2. Description of the Related Art
Generally, governmental requirements with respect to hazardous gas emissions from internal combustion engines have become increasingly tough in recent years. To reduce the emission of hazardous gases, a variety of technologies are now under development. When a catalytic converter is inactive, for example, during a cold start of an internal combustion engine, a lot of hazardous gases are discharged. It is known that the three-way catalyst, which is a popular material for catalytic converters, exhibits its highest performance when it is in an active state and the air-to-fuel ratio is maintained at its theoretical value. However, since the catalytic converter is not activated at cold start, the hazardous gases produced by combustion are not decomposed and are discharged, as combusted, into the atmosphere, this results in a poor quality of exhaust gas being discharged into the atmosphere.
Japanese Unexamined Patent Application Publication No. Hei. 11-336574 discloses a technology for reducing hydrocarbon (HC) gases which are part of the hazardous gases. It converts post-combustion gas, after being discharged to the exhaust line, back to the cylinder of the internal combustion engine by controlling the duration of the time that both intake and exhaust valves are open at the same time, which is called the valve overlap.
It is possible to burn the unburned HCs in the post-combustion gas and thereby reduce HC emissions by recirculating the post-combustion gas, once it is discharged to the exhaust line, back into the cylinder. If a lot of post-combustion gas is returned to the cylinder, however, the state of combustion becomes unstable because of the increased ratio of the post-combustion gas to the pre-combustion gas. The technique disclosed in Japanese Unexamined Patent Application Publication No. Hei. 11-336574 sets the valve overlap at a value below a predetermined level in order to prevent the amount of post-combustion gas being recirculated to the cylinder from increasing. Then, since the period of time that both intake and exhaust valves are open is shortened, the amount of recirculated gas can be properly limited to avoid the above problem.
Note that the amount of combustion gas that will be returned is determined by the degree of valve overlap and pressure difference between the intake line and the exhaust line. If the pressure in the intake line does not differ much from that in the exhaust line, a sufficient amount of exhaust gas is not returned to the cylinder. As a result, the effect of burning yet-burned HCs is not fully achieved.
This is particularly a problem during engine starting. At the moment the engine is started, the engine has not reached its idle speed, and therefore the pressure in the intake line is not low enough to pull post-combustion gas back into the cylinder. It then becomes impossible to take in a sufficient amount of post-combustion gas during engine starting by the technology disclosed in Japanese Unexamined Patent Application Publication No. Hei. 11-336574 that introduces combustion gas back into the cylinder by retarding the closure of the exhaust valve by controlling valve overlap. Particularly during cold start, unburned HCs are discharged into the air as they are because the catalytic converter has not yet been activated.
Part of the fuel injected during a cold start is not involved in combustion, and does not burn well in the cylinder but adheres to the cylinder wall. This is because fuel is not sprayed into the cylinder very well upon cold start, compared with that sprayed during normal engine operation. Since the fuel lodged onto the cylinder walls is less exposed to air, it is further less involved in combustion. Such unburned HC adhering to the cylinder wall is scraped up by the piston and discharged into the exhaust line when the exhaust valve opens. This HC is referred to as quench HC.
An object of the present invention is therefore to provide a valve timing control device for internal combustion engines in order to reduce the emissions of unburned HC.
Another object of the invention is to provide a valve timing control device that can reduce the amount of unburned HC discharged into the exhaust line including during a cold start.
According to a first aspect of the present invention, a valve timing control device for an internal combustion engine includes an exhaust valve control means for controlling at least a close timing of an exhaust valve installed in an internal combustion engine and a first engine condition detection means for detecting or estimating a condition of the internal combustion engine. The exhaust valve control means advances or retards the exhaust valve close timing from an intake top dead center based on the engine condition detected or estimated by the engine condition detection means.
It then becomes possible to retain post-combustion gas in the combustion chamber by advancing the close timing of the exhaust valve when the first engine condition detection means detects an engine condition under which a sufficient amount of post-combustion gas cannot be returned to the combustion chamber. If the post-combustion gas is retained in the combustion chamber, it becomes possible that unburned hydrocarbons in the combustion gas (hereafter called unburned HCs) will be involved in combustion during the next combustion round. Therefore, hazardous gas emissions are reduced. Meanwhile, when a sufficient amount of post-combustion gas can be recirculated into the combustion chamber by retarding the exhaust valve close timing, it becomes possible to reduce unburned HC emissions by reintroducing the unburned HCs in combustion. The above approach can accomplish the first goal of reducing emissions by cutting unburned HC.
According to a second aspect of the present invention, the first engine condition detection means detects or estimates pressure in an intake line or a parameter related to pressure as the operation condition of the internal combustion engine. The exhaust valve control means advances the exhaust value close timing from intake top dead center when the pressure or parameter detected or estimated by the first engine condition detection means is larger than a predetermined value while retarding the exhaust value close timing from the intake top dead center when the pressure or parameter detected or estimated by the first engine condition detection means is smaller than a predetermined value.
Then it becomes possible to advance or retard exhaust valve closure based on the pressure in the intake line or a parameter related to the pressure. Specifically, when the pressure difference between the intake line and the exhaust line is small, the close timing of the exhaust valve is advanced to confine the post-combustion gas in the combustion chamber to burn the unburned HC. On the other hand, when the pressure difference between the intake line and the exhaust line is large, the combustion gas discharged in the exhaust line can be recirculated to the combustion chamber. Then the unburned HC is consumed in combustion, thereby reducing emissions. The above configuration can also accomplish the first goal of reducing emissions by cutting unburned HC.
According to a third aspect of the present invention, the valve timing control device may further include a second engine condition detection means for detecting the condition of the internal combustion engine. The second engine condition detection means estimates an amount of wet fuel adhering to the intake line and/or an amount of unburned hydrocarbons adhering to a cylinder wall of the internal combustion engine. The wet fuel and hydrocarbons are included in fuel injected from an injector supplying fuel to the internal combustion engine. The exhaust valve control means sets a degree of advance for the exhaust valve close timing from the intake top dead center based on the estimated amount of the wet fuel and/or unburned hydrocarbons adhering to the cylinder wall when advancing the exhaust value close timing based on the condition of the internal combustion engine.
Then it becomes possible to estimate the amount of wet fuel and unburned HC that are produced upon cold starting and that adheres to the cylinder wall, or becomes quench HC, even when a lot of such HC and wet fuel are produced. Because the exhaust valve closure timing can be advanced with reference to the estimates, the amount of post-combustion gas retained in the combustion chamber can be optimized. Then, since the unburned HC in the combustion gas is involved in combustion again, the emission of unburned HC from the engine can be cut.
It is a well-known technology to open both the intake valve and the exhaust valve in order to recirculate post-combustion gas, once discharged to the exhaust line, to the combustion chamber. The amount of combustion gas to be returned, however, is determined by a pressure difference between the exhaust line and the intake line. Thus, if the pressure in the intake line is near atmospheric pressure, and as a result, the pressure difference is small between the exhaust line and the intake line, it is theoretically difficult to return the unburned gas to the combustion chamber by opening both the exhaust valve and the intake valve.
In such a case, according to a fourth aspect of the present invention, if the first engine condition detection means indicates that it possible to recirculate combustion gas to the combustion chamber, the exhaust valve control means controls a degree of retarding the exhaust valve closing timing from the intake top dead center. This occurs so that a period of time of both the intake valve and the exhaust valve of the internal combustion engine are open agrees with a predetermined period when retarding the exhaust valve close timing from the intake top dead center position based on the condition of the internal combustion engine detected by the first engine condition detection means.
Then it becomes possible to set the closure timing of the exhaust valve based on the period of time both valves are open and to precisely control the amount of post-combustion gas that will be recirculated to the combustion chamber.
According to a fifth aspect of the present invention, the valve timing control device further includes an intake valve control means for controlling at least an intake valve open timing. The second engine condition detection means includes a combustion state detection means for detecting a combustion state of the internal combustion engine. The intake valve control means retards the intake valve open timing when the combustion state detection means detects a condition meeting requirements for unburned hydrocarbons in the exhaust line to oxidize. The exhaust valve control means advances the exhaust valve close timing when the combustion state detection means detects stable combustion.
When unburned hydrocarbons can be oxidized, the unburned hydrocarbons are consumed in oxidation in the exhaust line when unburned hydrocarbons are discharged to the exhaust line. Under such a condition, the control according to the fifth aspect of the present invention is useful to stabilize combustion. Specifically, if the open timing of the intake valve is retarded, the intake flow speed increases because of the pressure difference between the intake line and the combustion chamber. Then fuel can then be better sprayed and the state of combustion can be improved. Further, by advancing the close timing of the exhaust valve under the above condition, the amount of post-combustion gas once discharged to the exhaust line and then returned to the combustion chamber is reduced, which contributes to the improvement of combustion stability.
According to a sixth aspect of the present invention, the valve timing control device further includes an air-to-fuel ratio detection means for detecting an air-to-fuel ratio in the internal combustion engine. The device also includes an exhaust temperature detection means for detecting or estimating a temperature of exhaust gas discharged from the internal combustion engine. The combustion state detection means decides that the combustion state of the internal combustion engine is stable if the exhaust temperature detection means detects or estimates an exhaust temperature higher than a predetermined temperature. At the same time, the air-to-fuel ratio detection means detects a combustion gas air-to-fuel ratio falling within a predetermined lean range. The exhaust valve control means advances the exhaust valve close timing based on the detection results provided by the combustion state detection means.
The combustion gas discharged to the exhaust line includes unburned HC. The invention checks whether the unburned HC in the post-combustion gas can be oxidized in the exhaust line based on the air-to-fuel ratio and the temperature of the exhaust gas. When the unburned HC can be oxidized in the exhaust line, the close timing of the exhaust valve is advanced to reduce the amount of post-combustion gas that will be returned to the combustion chamber and thereby stabilize combustion.
As previously described, the combustion gas produced at cold start contains a lot of wet fuel and quench HC. The prior art technique controls the period of time both intake valve and exhaust valve are open so as to reduce such quench HC and wet fuel. There is, however, a concern that a sufficient amount of post-combustion gas in the exhaust line will not be returned to the combustion chamber. This occurs when a small pressure difference between the intake line and the exhaust line renders it difficult to re-circulate the combustion gas to the combustion chamber.
According to a seventh aspect of the present invention, a valve timing control device for an internal combustion engine includes an exhaust valve control means for controlling at least the close timing of the exhaust valve installed in the internal combustion engine and a second engine condition detection means for detecting or estimating the condition of the internal combustion engine. The exhaust valve control means advances the exhaust valve close timing from intake top dead center based on the engine condition detected or estimated by the second engine condition detection means since cold starting of the engine.
Then it becomes possible to confine the post-combustion gas rich in quench HC and wet fuel in the combustion chamber. Since such quench HC and wet fuel can be returned to combustion, the amount of unburned HC and its emission can be reduced. As a result, the second goal of the invention to cut unburned HC in the exhaust line, even during cold starting, can be accomplished.
According to an eighth aspect of the present invention, the second engine condition detection means estimates the amount of wet fuel adhering to the intake line and/or the amount of unburned hydrocarbons adhering to the cylinder wall of the internal combustion engine, the wet fuel and hydrocarbons being included in the fuel injected from the injector supplying fuel to the internal combustion engine. The exhaust valve control means sets the degree of advance of the exhaust valve close timing from the intake top dead center position based on the estimated amount of the wet fuel and/or unburned hydrocarbons adhering to the cylinder wall when advancing the exhaust value close timing from the intake top dead center position based on the condition of the internal combustion engine detected by the second engine condition detection means.
Then it becomes possible to estimate the amount of wet fuel and unburned HC, or quench HC, produced upon cold starting of the engine and adhered to the cylinder wall, even when a lot of quench HC and wet fuel are produced. Because the exhaust valve close timing is advanced with reference to the estimates, the amount of combustion gas confined in the combustion chamber can be raised. As a result, since the unburned HC in post-combustion gas is involved in combustion again, the emission of unburned HC from the engine can be cut and emission quality can be improved.
According to a ninth aspect of the present invention, a valve timing control device for an internal combustion engine includes an exhaust valve installed in the internal combustion engine and an exhaust valve control means for controlling at least the close timing of the exhaust valve. Furthermore, the exhaust valve control means advances the exhaust valve close timing from the intake top dead center position upon cold starting of the internal combustion engine.
Then it becomes possible to confine the post-combustion gas rich in unburned HC in the combustion chamber. The unburned HC is returned to the combustion chamber even if the pressure difference between the intake line and the exhaust line is small. As a result, the amount of HC and its emission can be reduced.
According to a tenth aspect of the present invention, the valve timing control device further includes a combustion gas treatment means that is installed in the exhaust line of the internal combine engine and treats combustion gas discharged from the internal combustion engine. Additionally, a treatment capability detection means is provided for detecting or estimating the combustion gas treatment capability of the combustion gas treatment means. The exhaust valve control means advances the exhaust valve close timing from the intake top dead center position upon starting of the internal combustion engine when the treatment capability detection means detects a low combustion gas treatment capability in the combustion gas treatment means.
Unless the combustion gas treatment means has a sufficient combustion gas treatment capability, there is concern that part of the unburned HC produced during combustion may be discharged into the air, that is, with no treatment. Thus, in order to reduce the amount of unburned HC, the close timing of the exhaust valve is advanced beyond the intake top dead center position. This makes it possible to confine post-combustion gas in the combustion chamber and to have the unburned HC included in the post-combustion gas again undergo combustion. As a result, the emission of unburned HC is reduced and the quality of exhaust gas is improved.
There are two methods for reducing unburned HC included in post-combustion gas. As previously described, the combustion gas may be confined in the combustion chamber, and alternatively the post-combustion gas may be returned to the combustion chamber. When the pressure difference between the intake line and the exhaust line is small, it is difficult by the latter method to return a sufficient amount of post-combustion gas to combustion because this method relies on a pressure difference between intake and exhaust lines. Specifically, since the pressure in the intake line of the internal combustion engine is close to atmospheric pressure upon engine starting, it is difficult to re-circulate the discharged combustion gas to the combustion chamber.
Thus according to an eleventh aspect of the present invention, the exhaust valve control means retards the exhaust valve close timing from the intake top dead center position when a predetermined time has passed since the start of the internal combustion engine.
After the predetermined period of time that the rotation speed of the internal combustion engine has increased, the pressure in the intake line becomes increasingly low and the pressure difference from the exhaust line becomes increasingly large. Then it becomes possible to return the post-combustion gas to the combustion chamber and thereby to reduce the emission of unburned HC. The above predetermined period may be modified by the rotational speed of the internal combustion engine, accumulated rotations, frequency of ignitions, accumulated amount of intake air and other factors.
According to a twelfth aspect of the present invention, a valve timing control device for an internal combustion engine includes an exhaust valve installed in the internal combustion engine and an exhaust valve control means for controlling at least the close timing of the exhaust valve. The exhaust valve control means advances the exhaust valve close timing from the intake top dead center position when a desired amount of combustion gas, once discharged from the internal combustion engine to the exhaust line, cannot be recirculated into the cylinder of the internal combustion engine.
Then, even when a desired amount of post-combustion gas cannot be returned, the post-combustion gas is retained in the combustion chamber so as to have unburned HC discharged from the internal combustion engine involved in the next round of combustion. As a result, the amount of unburned HC is reduced in the combustion gas discharged from the internal combustion engine, and the degradation of exhaust gas quality can be prevented.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.